An Ultrastructural Study on the Development of Central Cells in Rice Embryo Sac

Abstract

An ultrastructural study on the development of the central cell was carried out on rice (Oryza sativa L.). The results showed that following the formation of the eight nuclei in the embryo sac, cellularization took place resulting in the formation of seven cells. One of the cells situated in the centre of the embryo sac was the central cell. The central cell contained a large vacuole, two polar nuclei (located in micropylar part and chalzal end, respectively) and a large number of cytoplasmic organella. Further development of the central cell mainly involved changes in the orientation of the polar nuclei and the distribution of the cytoplasm. Changes in the orientation of the polar nuclei involved the following stages of developments a. The two polar nuclei increased in size and became elliptical in shape. On both sides of the polar nuclei the cytoplasm showed an asymmetrical pattern of distribution. b. The two polar nuclei moved to the central region of embryo sac. At this stage the two polar nuclei were close to each other and they were lying ('one on top of the other') in parallel to the long axis of the embryo sac. When the polar nuclei moved to the central region of the embryo sac, a thick strand of cytoplasm (forming a cytoplasmic 'bridge' ) became associated with the polar nuclei. The cytoplasm 'bridge' ran from one end of the embryo sac to the other. c. The two polar nuclei began to move towards the egg cell and finally 'sat' themselves on the top of the egg cell. The two polar nuclei had by now changed from a parallel (one on top of the other) to perpendicular (side by side) orientation (with respect to the long axis of the embryo sac). The side of the nuclear envelope of the two polar nuclei facing each other produced a lot of protrusions and fusion bridges throughout the different stages of development and reorientation of the polar nuclei. The cytoplasm surrounding the polar nuclei also went through some changes e. g. starch grains disappeared and smooth ER increased. UP till stage c the size of the embryo sac did not change much. But immediately after the completion of stage c the embryo sac greatly increased in size. The embryo sac then reached full maturity and apparently did not undergo any further changes.

本文通過透射電鏡對水稻受精前胚囊中央細胞發育過程中超微結構的變化進行觀察。結果表明，八核胚囊形成后很快就進行細胞化形成7個細胞，其中剛形成的中央細胞由1個大液泡、2個極核（珠孔端和合點端各1個）和一些含有豐富細胞器的胞質組成。中央細胞以后的發育主要是極核的發育和極核周圍胞質的變化。極核發育經歷以下過程：a．2個核都膨大呈“橢圓”形。核周圍胞質呈不對稱分布。b．2個核分別向胚囊中央移動并相互靠近。之后2個極核調整排列方式，由縱排（即與胚囊縱軸平行）變成橫排。此時期有細胞質“橋”聯結珠孔端卵器、2個極核和合點端反足細胞器。c．橫排的極核移向卵器，并排列于卵細胞之上。此時胚囊未明顯膨大，但極核相靠近的兩邊核膜有許多處已形成“融合橋”，核周圍的胞質也起較大的變化，如質體內淀粉消失和光面內質網增加等。極核進一步發育直至胚囊成熟期間，極核排列方式及其周圍胞質組成未觀察到明顯的變化，但胚囊體積明顯增大。